This site gives information on HTML elements, attributes, element categories, and date/time formats.
This site gives information on CSS rule syntax, selectors, combinators, pseudo classes, and pseudo elements. The site also lists different CSS properties.
The MDN HTML and CSS pages give basic information on each language for anyone that wants to learn more and/or look for coding help. Trusted resources are important in looking for programming help because you want to ensure yourself that you're getting accurate, up-to-date information so you don't have faulty code.